What if extraterrestrial life is hiding in places we had completely ruled out? For decades, astronomers have focused their search on a narrow band around stars, where water could be liquid on the surface. However, this traditional approach is being challenged by recent studies.
Indeed, new climate simulations indicate that worlds much closer or much farther from their star could harbor environments conducive to life. These models take into account planets that always show the same face to their sun, a phenomenon called synchronous rotation. On these bodies, the side perpetually plunged into night could retain liquid water thanks to efficient heat redistribution by the atmosphere or an ocean.
Within our own Solar System, life could exist beyond the habitable zone.
Representation of hydrothermal activity on Enceladus based on data from the Cassini-Huygens mission
Credit: ESA These planets, common around M-type dwarf stars, could thus be located closer to their star without their water completely evaporating. This idea is supported by recent observations from the James Webb Space Telescope, which detected water vapor in the atmosphere of some exoplanets located inside the traditional limit of the habitable zone.
Furthermore, the widening of the boundaries of the habitable zone does not only concern the inner limit. Even distant, icy planets could hide liquid water under thick layers of ice, heated by the planet's interior. On our own planet, subglacial lakes like those in Antarctica harbor microbial life, proving that surface water is not essential.
This reassessment of climate models opens considerable perspectives for the search for life in the Universe. It multiplies the number of worlds to study and questions our criteria for habitability. The research team, whose work was published in the
Astrophysical Journal, thus proposes to review the zones where life could emerge.
Future telescope observations will be able to test these new hypotheses by scrutinizing the atmospheres of exoplanets located outside classic zones. This broader approach could lead us to discover biological signatures where we did not expect them.
Synchronous rotation and its climatic effects
Many exoplanets, especially those orbiting red dwarf stars, are gravitationally locked. This means they rotate on their axis in exactly the same time it takes them to orbit their star. Consequently, one face is in perpetual, scorching day, while the other is plunged into an eternal, icy night.
For a long time, it was thought that this configuration prevented any habitable conditions. The extreme temperature of both hemispheres seemed destined to cause the atmosphere to collapse or disappear. The absence of a day-night cycle also seemed unfavorable for climate stability.
However, three-dimensional climate models have shown that a sufficiently dense atmosphere can transport heat from the day side to the night side. A global ocean would play a similar role by redistributing thermal energy through currents. This can create a temperate band at the boundary between day and night, or even maintain bearable temperatures over a large part of the night-side surface.
This discovery is important because red dwarf stars are the most common in the Galaxy. Their synchronously rotating planets therefore represent a prime target for the search for life, provided we look in the right place.
Oceans under the ice, little-known refuges
Far from its star, a planet receives very little heat. Its surface should be a global, thick, and solid ice sheet. Yet, life could thrive not on the surface, but deep below, beneath a thick layer of ice that would act as a thermal insulator.
The heat needed to keep water in a liquid state can come from the planet's interior itself. The decay of radioactive elements in the core and mantle generates energy. Tidal forces, exerted by the star or by massive bodies, can also cause friction and heat the planet's interior, a phenomenon observed on some moons of Jupiter and Saturn.
On Earth, entire ecosystems exist in the total darkness of lakes beneath the Antarctic ice sheet, like Lake Vostok. These environments, cut off from the surface and sunlight for millions of years, are rich in microbes that derive their energy from chemical reactions, not photosynthesis.
These icy ocean worlds could be numerous in our galaxy. Studying them forces us to broaden our definition of habitability beyond the simple presence of surface liquid water, to include these vast buried reservoirs, protected from stellar radiation and extreme climate variations.